ParseXS: refactor: remove stray $_

(This commit is part of a series which will extend the AST parse tree
from just representing individual XSUBs to representing the whole XS
file.)

Remove this line:

    $_ = '';

which is a leftover from when $_ used to maintain the current line
between calls to various parsing subs. It's redundant now.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS/Node.pm

@@ -1782,8 +1782,6 @@ sub parse {
         or return;
     push @{$self->{kids}}, $decl;
 
-    $_ = '';
-
     # Check all the @{ $pxs->{line}} lines for balance: all the
     # #if, #else, #endif etc within the XSUB should balance out.
     ExtUtils::ParseXS::Utilities::check_conditional_preprocessor_statements();