Remove PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES

We will deprecate flattened apexes. In this change, GSI-specific make
variable (PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES) is removed. The
variable was used to install both image/flattened apexes in the GSI, so
that it works on ro.apex.updatable devices and not-updatable devices.

Now, GSI will have only image APEXes in it.

Bug: 278826656
Test: lunch gsi_arm64-userdebug && m # no flattened apexes
Change-Id: I4702973d4ee75aa693e4e7f4e57577b77059dc09