Remove PRODUCT_BOOT_JAR_MODULE_OVERRIDES.

This reverts commit efe6a4d7.

As a result of b/191269918, APEX variants are now consistently
identified by their "runtime names", i.e. their mount names under
/apex. Those names are now also used to identify the APEXes in
PRODUCT_BOOT_JARS and similar variables. That avoids implementing a
global lookup mechanism in Soong, and since they don't vary between
products also makes this override variable unnecessary.

Test: `m nothing` in internal
Bug: 191269918
Bug: 180325915
Change-Id: I6fd3d29d1c032c9f8bda0191781f9d2dc6f199a4