Remove apex.apex_name

apex_name was used to install symbol files under /apex/<apex name>,
which caused duplicate rules when there's two apexes with the same
apex_name. (e.g. override_apex or multi-install apexes)

Since symbol files can be looked up via ELF Build-Id, we don't have to
install symbol files with the actual "apex name", but instead we can
install them with soong module name.

Bug: 267581665
Test: m (soong test)
Change-Id: I1cbe5c86d287f180c7deef93ba85ea5552680011