Delete unused variables from clear_vars.mk

Kati analysis in AOSP spends around 6 seconds in clear_vars.mk.
Delete any variables in clear_vars.mk that are not referenced anywhere
else in build/make/core.

Test: no change to build-aosp_cf_x86_64_phone.ninja
Change-Id: I7e0db3c02d297de825acbfbd1a0f05724d1e846d