Remove apex.multi_install_skip_symbol_files

This has been a workaround to avoid duplicate build rules for
multi-install apexes. The reason for duplicate errors was that apex_name
was used to install symbol files.

We don't need to rely on apex_name for symbol files, hence no need for
the workaround.

Note that symbol files for apexes are still available via the paths
based on "soong module name".

Bug: 267581665
Test: cuttlefish (which uses multi-apex) build & run
Change-Id: Icaca667e139ef2aca32339a75b54b12a00449c2d