[DynInst_API:] [dyninst/dyninst] 80f84e: Remove ret_lock from SymtabAPI::FunctionBase (#1504)

[Tim Haines <noreply@xxxxxxxxxx>]

  Branch: refs/heads/master
  Home:   https://github.com/dyninst/dyninst
  Commit: 80f84e4f2e27d91929f48290740899c3edb3b77b
      https://github.com/dyninst/dyninst/commit/80f84e4f2e27d91929f48290740899c3edb3b77b
  Author: Tim Haines <thaines.astro@xxxxxxxxx>
  Date:   2023-09-06 (Wed, 06 Sep 2023)

  Changed paths:
    M symtabAPI/h/Function.h
    M symtabAPI/src/dwarfWalker.C

  Log Message:
  -----------
  Remove ret_lock from SymtabAPI::FunctionBase (#1504)

This was used to guard calling setReturnType. That function is only
ever called from DwarfWalker::setFuncReturnType which is itself
guarded by `ParsedFuncs::accessor` in DwarfWalker::parseSubprogram,
rendering this mutex unnecessary.