build: drop obsolete yacc/lex dist hack (fix out of source builds)

f60f46da prevented generated yacc/lex files from being included in the tarball which confuses automake and breaks out-of-source (VPATH) builds.

Let's drop the workaround as it doesn't seem to make much sense now and the automake developers explicitly said it should be removed.

Bug: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=54390 Closes: #8