[TableGen] Delete support for deprecated positional matching.

After the work in a538d1f1 5351878b 372240df, and
subsequently cleanup of all the in-tree targets, we can now delete the
support for positional operand matching!

This removes three options which could previously be set in a
Target's "InstrInfo" tablegen definition:
- useDeprecatedPositionallyEncodedOperands
- decodePositionallyEncodedOperands
- noNamedPositionallyEncodedOperands

(Also announced at https://discourse.llvm.org/t/tablegen-deleting-deprecated-positional-instruction-operand-matching-support/68524)

Differential Revision: https://reviews.llvm.org/D144210