Drop useless "const" qualifiers from Perl-to-APR array converters.

What good does a return type of "const apr_array_header_t *" do when
all callers cast the value to just "apr_array_header_t *"?
Besides, neither the similar Perl-to-APR hash converters do this
nor the Ruby equivalents.

* subversion/bindings/swig/include/svn_containers.swg:
  Remove casts from the calls to svn_swig_pl_*_to_array().

* subversion/bindings/swig/perl/libsvn_swig_perl/swigutil_pl.c,
  subversion/bindings/swig/perl/libsvn_swig_perl/swigutil_pl.h:
  Drop the "const" from the return types of svn_swig_pl_*_to_array().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1482420 13f79535-47bb-0310-9956-ffa450edef68