Remove special handling of frameworks/base/config/boot-profile.txt

Previously, the method was run in repo manifests that did not include
the frameworks/base repository so it had to handle the file being
missing. However, now that this is being called from
platform_bootclasspath which is only defined in frameworks/base that
is no longer required.

Bug: 177892522
Test: m droid
Change-Id: I77fa5a204d1426a6be24a6f0b48e18f48f3dd908