Remove redundant definitions of M_ARENA_* macros

The M_ARENA_MAX and M_ARENA_TEST macros are defined in malloc.c as
well as malloc.h, and the former is unnecessary.  This patch removes
the duplicate.  Tested on x86_64 to verify that the generated code
remains unchanged barring changed line numbers to __malloc_assert.

	* malloc/malloc.c (M_ARENA_TEST, M_ARENA_MAX): Remove.