[lldb][NFC] Remove unused ExpressionParser::Parse

Summary:
This function is only used internally by ClangExpressionParser. By putting it in the ExpressionParser class all languages
that implement ExpressionParser::Parse have to share the same signature (which forces us in downstream to add
swift-specific arguments to ExpressionParser::Parse which then propagate to ClangExpressionParser and so on).

Reviewers: davide

Subscribers: JDevlieghere, lldb-commits

Tags: #upstreaming_lldb_s_downstream_patches, #lldb

Differential Revision: https://reviews.llvm.org/D69710