perfglue/heap_profiler.cc: expect args as first element on cmd vector

perfglue/heap_profiler.cc: expect args as first element on cmd vector

We used to pass 'heap' as the first element of the cmd vector when
handling commands.  We haven't been doing so for a while now, so we
needed to fix this.

Not expecting 'heap' also makes sense, considering that what we need to
know when we reach this function is what command we should handle, and
we should not care what the caller calls us when handling his business.

Fixes: #6361
Backport: dumpling

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>