gitignore: remove non-existing 'doc/manuals/osmomsc-usermanual.xml'

Change-Id: If0fef3f12f15780ed3a3e33db25cd29082ff142a
diff --git a/.gitignore b/.gitignore
index 43fdfc2..2020422 100644
--- a/.gitignore
+++ b/.gitignore
@@ -67,7 +67,6 @@
 doc/manuals/vty/osmotrx-*-vty-reference.xml
 doc/manuals/vty/osmotrx-*-vty-reference.xml.inc.gen
 doc/manuals/vty/osmotrx-*-vty-reference.xml.inc.merged
-doc/manuals/osmomsc-usermanual.xml
 doc/manuals/common
 doc/manuals/build