Remove valgrind related code from tcache

In jemalloc.c we already call VALGRIND_MALLOCLIKE_BLOCK. In addition,
malloc_conf_init appears to disable opt_tcache if RUNNING_ON_VALGRIND
is true.

Removing this code saves about 10% of the bytes in the body of malloc
in addition, it saves about 224 bytes (65%) of the stack space malloc
reserves.