cmd/compile/internal/types: remove unused lineno arguments for PushDcl/MarkDcl
    
    More steps towards simpler symbol handling:
    
    - Pushdcl's incoming pos argument, saved in a newly pushed *Sym, was always
      immediately overwritten by the Lastlineno value of the saved *Sym.
    
    - Markdcl's incoming pos argument, saved in the stack mark *Sym, was not
      restored when the stack mark was popped.
    
    - Popdcl always maintained the most recent Lastlineno for a *Sym given
      by package and name, making it unnecessary to save Lastlineno in the
      first place. Removed Lastlineno from the set of fields that need saving,
      and simplified Popdcl.
    
    Change-Id: Ie93da1fbd780dcafc2703044e781c0c6298df569
    Reviewed-on: https://go-review.googlesource.com/41390
    Run-TryBot: Robert Griesemer <gri@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    Reviewed-by: Matthew Dempsky <mdempsky@google.com>