cmake: remove unused STDCALL option

The `STDCALL` option was removed; remove the (unreachable) error
message.

Author: ethomson

CMakeLists.txt

@@ -97,10 +97,6 @@ ENDIF()
 
 # Platform specific compilation flags
 IF (MSVC)
-	IF (STDCALL)
-		MESSAGE(FATAL_ERROR "The STDCALL option is no longer supported; libgit2 is now always built as a cdecl library.  If you're using PInvoke, please add the CallingConventions.Cdecl attribute for support.")
-	ENDIF()
-
 	ADD_DEFINITIONS(-D_SCL_SECURE_NO_WARNINGS)
 	ADD_DEFINITIONS(-D_CRT_SECURE_NO_DEPRECATE)
 	ADD_DEFINITIONS(-D_CRT_NONSTDC_NO_DEPRECATE)