build: use a static .tiff for macOS .dmg over generating

Co-authored-by: Pavol Rusnak <pavol@rusnak.io>

.gitignore

@@ -96,7 +96,6 @@ Makefile
 !depends/Makefile
 src/qt/bitcoin-qt
 Bitcoin-Qt.app
-background.tiff*
 
 # Qt Creator
 Makefile.am.user
@@ -149,6 +148,5 @@ db4/
 
 osx_volname
 dist/
-*.background.tiff
 
 /guix-build-*