Perl: removed unneeded NGX_DONE test.

The NGX_DONE test in ngx_http_perl_handle_request() was introduced
in 1702:86bb52e28ce0, which also modified ngx_http_perl_call_handler()
to return NGX_DONE with c->destroyed.  The latter part was then
removed in 3050:f54b02dbb12b, so NGX_DONE test is no longer needed.