Replace Ghostscript DOCINFO and fix 9.25 metadata date regression

We no longer use Ghostscript to manage PDF metadata, instead
omitting the DOCINFO segment from the pdfmark file we generate.

Instead all of the relevant metadata code has been migrated to pikepdf,
and we use that API. This should be more consistent and fixes the
Ghostscript version-depedent quirks.

Also removes our python-xmp-toolkit dependency, except for
testing.

tests/test_metadata.py

@@ -94,9 +94,6 @@ def test_override_metadata(spoof_tesseract_noop, output_type, resources,
     before = pikepdf.open(input_file)
     after = pikepdf.open(outpdf)
 
-    if ghostscript.version() >= '9.24':
-        pytest.xfail('Ghostscript 9.24+ does not support Unicode DOCINFO')
-
     assert after.metadata.Title == german, after.metadata
     assert after.metadata.Author == chinese, after.metadata
     assert after.metadata.get('/Keywords', '') == ''
@@ -166,11 +163,7 @@ def test_creation_date_preserved(spoof_tesseract_noop, output_type, resources,
     after = pdf_after.trailer.get('/Info', {})
 
     if not before:
-        # If there was input creation date, none should be output
-        # because of Ghostscript quirks we set it to null
-        # This test would be better if we had a test file with /DocumentInfo but
-        # no /CreationDate, which we don't
-        assert after.get('/CreationDate', '') == ''
+        assert after.get('/CreationDate', '') != ''
     else:
         # We expect that the creation date stayed the same
         date_before = decode_pdf_date(str(before['/CreationDate']))