remove primitive conversion challenge

Author: cvick32

doc/src/SUMMARY.md

@@ -26,4 +26,4 @@
   - [11: Safety of Methods for Numeric Primitive Types](./challenges/0011-floats-ints.md)
   - [12: Safety of `NonZero`](./challenges/0012-nonzero.md)
   - [13: Safety of `CStr`](./challenges/0013-cstr.md)
-  - [14: Safety of Primitive Conversions](./challenges/0014-convert-num.md)
+