Don't remove entries for overriddable modules

Previously, when there is apex_set that is overriding another module,
the entry for the overridden module is removed from apexkeys.txt.
However, this is wrong because the existence of the apex_set module
doesn't necessary mean that the module is insatalled instead of the
overridden module. That is determined by PRODUCT_PACKAGES which Soong
has no knowledge of. Therefore, we don't delete the entry for the
overridden (actually possibly overridable) modules in the file.

Bug: 158729168
Test: m
Change-Id: I85d0c756f862323bae556bf657d66ec50038985f