tests/mem_alloc: Let picolibc pick a malloc heap size

Now that picolibc's malloc arena configuration always allocates
some space, we don't need explicit allocations for tests.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

tests/lib/mem_alloc/prj_picolibc.conf

@@ -1,4 +1,3 @@
 CONFIG_ZTEST=y
 CONFIG_PICOLIBC=y
-CONFIG_PICOLIBC_ALIGNED_HEAP_SIZE=8192
 CONFIG_TEST_USERSPACE=y