Speed up maybe_extract_lines and removed unused variables

using these test results #115 (comment)

Author: cdeler

h11/_readers.py

@@ -20,7 +20,6 @@
 
 from ._abnf import chunk_header, header_field, request_line, status_line
 from ._events import *
-from ._receivebuffer import line_delimiter_regex
 from ._state import *
 from ._util import LocalProtocolError, RemoteProtocolError, validate
 

h11/_receivebuffer.py

@@ -42,7 +42,10 @@
 # than the internal bytearray support.)
 
 blank_line_delimiter_regex = re.compile(b"\n\r?\n", re.MULTILINE)
-line_delimiter_regex = re.compile(b"\r?\n", re.MULTILINE)
+
+
+def rstrip_line(line):
+    return line.rstrip(b"\r")
 
 
 class ReceiveBuffer(object):
@@ -126,6 +129,6 @@ def maybe_extract_lines(self):
             if data is None:
                 return None
 
-            lines = line_delimiter_regex.split(data.rstrip(b"\r\n"))
+            lines = list(map(rstrip_line, data.rstrip(b"\r\n").split(b"\n")))
 
             return lines