misc cleanups for symbol-check

Summary:
Remove no longer needed Xenial related symbols.

The other change is a follow-up to D13971 and D13892: check_PE_subsystem_version does not take filename as an argument, and the current code only works by accident, because filename happens to be defined to the correct value in the outer scope when this function is called.

This is partially a backport of core#25392

Test Plan:
gitian builds
contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15578