Remove various rarely used test targets from the generated Makefile.

Remove various rarely used test targets from the generated Makefile.

Remove these targets and their documentation:

check.third check.utf16 check.utf8 coretest minitest.utf16 test.deparse
test.taintwarn test.third test.torture test.utf16 test.utf8
test_notty.deparse test_notty.third test_prep.third torturetest ucheck
ucheck.third ucheck.utf16 ucheck.valgrind utest utest.third utest.utf16
utest.valgrind

It's still possible to run the actions these targets "by hand", if desired.
This commit simply removes the convenience targets from the Makefile,
reducing its complexity.