Revert half of "ANDROID: vendor_hooks: Add hooks for memory when debug"

This reverts parts of commit 24149445.

The hooks android_vh_alloc_pages_slowpath,
android_vh_print_slabinfo_header, and android_vh_cache_show are not used
by any vendor, so remove them to help with merge issues with future LTS
releases.

If this is needed by any real user, it can easily be reverted to add it
back and then the symbol should be added to the abi list at the same
time to prevent it from being removed again later.

Bug: 182443489
Cc: Liujie Xie <xieliujie@oppo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I63621110c0c38b7e5d5e5e6f85c513bf00ecc00b