tests/kernel/common: 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/kernel/common/testcase.yaml

@@ -32,4 +32,3 @@ tests:
     tags: picolibc
     extra_configs:
       - CONFIG_PICOLIBC=y
-      - CONFIG_PICOLIBC_ALIGNED_HEAP_SIZE=8192