Revert "lib/os/heap: introduce option to force big heap mode"

This reverts commit b6b6d39b.

With both commit 4690b8d5 ("libc/minimal: fix malloc() allocated
memory alignment") and commit c822e0ab ("libc/minimal: fix
realloc() allocated memory alignment") in place, there is no longer
a need for enforcing the big heap mode on every allocations.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>