PdfViewer: Drop git submodule

Signed-off-by: Mohammad Hasan Keramat J <ikeramat@protonmail.com>
Change-Id: I13af3570df91e1cc0dbcbc6da7e44ce6d93f9068

.gitmodules

-[submodule "pdfjs-dist"]
-	path = third_party/pdfjs-dist
-	url = https://github.com/mozilla/pdfjs-dist.git

pdfjs-dist @ cc3d3bf2

-Subproject commit cc3d3bf299ae11f8f72ae8d64cbf19b340f9a996