bsdgrep(1): Rip out "xmalloc" bits

xmalloc was a debug malloc implementation, but the x{malloc,calloc,free}
functions default to calling the malloc(3) equivalents.

Instead of relying on this malloc shim, we can devise better ways to debug
malloc issues that aren't misleading upon initial inspection. (I.e., using
jemalloc's various built-in debugging capabilities.)

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: emaste, cem
Differential Revision: https://reviews.freebsd.org/D10269