ANDROID: GKI: Remove temp build file abi_gki_protected_exports

abi_gki_protected_exports is a prepped list of symbols protected
from being exported by unsigned modules; and input to generate
gki_module_protected_exports.h during kernel build.

Delete it once header is generated so it is not lingering in the
source when kernel sournce is being built in-place i.e. OBJ is
not set during the build.

Bug: 286529877
Test: Manual verification & TH
Change-Id: Ia06db62da03289b8f90917bcd302c81c8a4d31d2
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>