LibCore: Remove unused include MemMem
These instances were detected by searching for files that include
MemMem.h, but don't match the regex:
\\b(MemMem(?!\.h>)|bitap_bitwise|memmem|memmem_optional)\\b
These are the only symbols defined by MemMem.h.
In theory, one might use LibCPP to detect things like this
automatically, but let's do this one step after another.
Ben Wiederhake