TwinCAT3: remove *.xtv (project-specific mappings since 4026.xx)

Since TwinCAT 3.1 build 4026.xx, the *.xtv file stores project-specific
mappings and must be tracked in version control. Only the backup and
temporary variants stay ignored.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

TwinCAT3.gitignore

@@ -28,7 +28,6 @@ LineIDs.dbg.bak
 *.xti.bak
 *.xti.bk?
 *.xti.orig
-*.xtv
 *.xtv.bak
 *.xtv.bk?
 *.xt?.bk?