[clang] NFC: remove unused / untested workaround in pack deduction (#183875)
This snippet was part of what was introduced in
130cc445e46836b28defdce03b1adfdb16ddcf41
However, none of the existing tests require it, including the tests
added in that commit.
One of those tests had a FIXME which was fixed when we switched
frelaxed-template-template-args on by default as well.