ANDROID: Fix allmodconfig build with CC=clang

When GCC_PLUGIN_STRUCTLEAK was backported, a prompt text mysteriously
made its way into the Kconfig option. Because this option is not
dependent on GCC_PLUGINS, it could become enabled even when building
with "CC=clang allmodconfig", which is not correct. The option is
correctly selected by GCC_PLUGIN_STRUCTLEAK_BYREF_ALL so this prompt
text seems to be unnecessary.

This change also aligns the help text to match upstream, to match the
version that was claimed to have been backported.

Fixes: e0c6791d ("BACKPORT: security: Create "kernel hardening" config area")
Bug: 143965122
Test: make CC=clang allmodconfig && make -j
Change-Id: Ia9dc88ec1bbfd3950eda5a3eb698ecd41c7e0c9a
Signed-off-by: Alistair Delva <adelva@google.com>