libsyntax: Remove the obsolete ability to parse from substrings.

This was used by the quasiquoter.
auto merge of #11575 : pcwalton/rust/parse-substrs, r=alexcrichton

This was used by the quasiquoter.

r? @alexcrichton
Demote self to an (almost) regular argument and remove the env param.
Fixes #10667 and closes #10259.