Fix bugtracker #340: small PDF text drawn too bold in Adobe Acrobat

Qt 6 declares the PDF/X namespace (pdfxid) in the XMP metadata of every
PDF it writes, even when the file is not PDF/X. Adobe Acrobat then draws
small text too bold at some zoom levels. Qt only needs the declaration
for PDF/X-4 output, which QElectroTech never asks for.

Blank the declaration out after export, in place with spaces so no
offsets move, in both the export window and the --export-pdf path.
Diagnosis and the Acrobat testing by Alf, bugtracker #340.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/print/projectprintwindow.cpp

@@ -874,6 +874,7 @@ void ProjectPrintWindow::print()
 			// Convert URI link annotations into native internal GoTo/FitR
 			// actions so cross-references jump inside the document.
 			PdfLinks::convertUriToGoTo(pdfFile);
+			PdfLinks::removeUnusedPdfxNamespace(pdfFile);
 
 			this->close();
 		});