util: remove unused p_strcasecmp define

It was unused and only defined in posix.h
It did not exist in its Windows equivalent.

src/util/unix/posix.h

@@ -54,8 +54,6 @@ GIT_INLINE(int) p_fsync(int fd)
 #define p_send(s,b,l,f) send(s,b,l,f)
 #define p_inet_pton(a, b, c) inet_pton(a, b, c)
 
-#define p_strcasecmp(s1, s2) strcasecmp(s1, s2)
-#define p_strncasecmp(s1, s2, c) strncasecmp(s1, s2, c)
 #define p_vsnprintf(b, c, f, a) vsnprintf(b, c, f, a)
 #define p_snprintf snprintf
 #define p_chdir(p) chdir(p)