Remove functions deprecated in latest libssh master. (Resolves #10)

Addressing
skipping 'ssh/session.c' Cython extension (up-to-date)
building 'ssh.session' extension
gcc -pthread -Wno-unused-result -Wsign-compare -DDYNAMIC_ANNOTATIONS_ENABLED=1 -DNDEBUG -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -D_GNU_SOURCE -fPIC -fwrapv -fPIC -Ilibssh/include -I/usr/include/python3.7m -c ssh/session.c -o build/temp.linux-x86_64-3.7/ssh/session.o -O3
ssh/session.c: In function ‘__pyx_pf_3ssh_7session_7Session_46is_server_known’:
ssh/session.c:4782:9: warning: ‘ssh_is_server_known’ is deprecated [-Wdeprecated-declarations]
         __pyx_v_rc = ssh_is_server_known(__pyx_v_self->_session);
         ^~~~~~~~~~
In file included from ssh/session.c:569:
libssh/include/libssh/libssh.h:551:31: note: declared here
 SSH_DEPRECATED LIBSSH_API int ssh_is_server_known(ssh_session session);
                               ^~~~~~~~~~~~~~~~~~~
ssh/session.c: In function ‘__pyx_pf_3ssh_7session_7Session_118write_knownhost’:
ssh/session.c:9577:9: warning: ‘ssh_write_knownhost’ is deprecated [-Wdeprecated-declarations]
         __pyx_v_rc = ssh_write_knownhost(__pyx_v_self->_session);
         ^~~~~~~~~~
In file included from ssh/session.c:569:
libssh/include/libssh/libssh.h:549:31: note: declared here
 SSH_DEPRECATED LIBSSH_API int ssh_write_knownhost(ssh_session session);
                               ^~~~~~~~~~~~~~~~~~~
ssh/session.c: In function ‘__pyx_pf_3ssh_7session_7Session_120dump_knownhost’:
ssh/session.c:9686:9: warning: ‘ssh_dump_knownhost’ is deprecated [-Wdeprecated-declarations]
         __pyx_v__known_host = ssh_dump_knownhost(__pyx_v_self->_session);
         ^~~~~~~~~~~~~~~~~~~
In file included from ssh/session.c:569:
libssh/include/libssh/libssh.h:550:33: note: declared here
 SSH_DEPRECATED LIBSSH_API char *ssh_dump_knownhost(ssh_session session);
                                 ^~~~~~~~~~~~~~~~~~
gcc -pthread -shared -Wl,-z,relro -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -g build/temp.linux-x86_64-3.7/ssh/session.o -L/root/ssh-python/src/src -L/usr/lib64 -Wl,--enable-new-dtags,-R$ORIGIN/. -lssh -lpython3.7m -o /root/ssh-python/ssh/session.cpython-37m-x86_64-linux-gnu.so

Author: adelton

Diff for: ssh/c_ssh.pxd

@@ -366,7 +366,6 @@ cdef extern from "libssh/libssh.h" nogil:
     int ssh_init()
     int ssh_is_blocking(ssh_session session)
     int ssh_is_connected(ssh_session session)
-    int ssh_is_server_known(ssh_session session)
 
     int ssh_set_log_level(int level)
     int ssh_get_log_level()
@@ -528,8 +527,6 @@ cdef extern from "libssh/libssh.h" nogil:
         ssh_session session, unsigned int i, const char *answer)
     int ssh_userauth_gssapi(ssh_session session)
     const char *ssh_version(int req_version)
-    int ssh_write_knownhost(ssh_session session)
-    char *ssh_dump_knownhost(ssh_session session)
 
     void ssh_string_burn(ssh_string str)
     ssh_string ssh_string_copy(ssh_string str)