Core: Drop custom copymem/zeromem defines

We've been using standard C library functions `memcpy`/`memset` for these since
2016 with 67f65f6639.

There was still the possibility for third-party platform ports to override the
definitions with a custom header, but this doesn't seem useful anymore.

Backport of #48239.

core/pool_allocator.cpp

@@ -31,7 +31,6 @@
 #include "pool_allocator.h"
 
 #include "core/error_macros.h"
-#include "core/os/copymem.h"
 #include "core/os/memory.h"
 #include "core/os/os.h"
 #include "core/print_string.h"
@@ -42,7 +41,7 @@
 	do {                                                      \
 		void *_dst = &((unsigned char *)pool)[m_to_pos];      \
 		void *_src = &((unsigned char *)pool)[(m_entry).pos]; \
-		movemem(_dst, _src, aligned((m_entry).len));          \
+		memmove(_dst, _src, aligned((m_entry).len));          \
 		(m_entry).pos = m_to_pos;                             \
 	} while (0);