texlive/combine.nix: fix bugs in languages filter

Initial language.{dat,def} configuration files provided by
`texlive.hyphen-base` may declare languages that were not part of the
combined packages. Those are filtered out by a sed script that had few
problems:

1) The sed script was generated from a list of potentially non-unique
packages. Every repetition of a select and print clause would produce a
copy of a language declaration in the output file. This became a problem
for update to the 2018-final, the fmtutil would crash from too much
German hyphenation.
2) The select clauses were ambiguous: both '^% from hyphen' and
'^% from hyphen-welsh' will match a line 'from hyphen-welsh'.