ProjectFileDataProvider: disconnect from closing project's signals

This optimization almost halves the number of calls to
ProjectFileDataProvider::fileRemovedFromSet(), because
&IProject::fileRemovedFromSet signal is emitted for each project's file
while it is being closed.
1 job for !176 with fix-quickopen-crashes in 63 minutes and 12 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
failed #71300
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
[ 63%] Linking CXX static library ../../lib/libkdevlldb_static.a
[ 63%] Built target kdevgdb_static
[ 63%] Built target kdevlldb_static
[ 63%] Generating qrc_kdevcontextbrowser.cpp
Scanning dependencies of target kdevcontextbrowser
[ 63%] Building CXX object plugins/contextbrowser/CMakeFiles/kdevcontextbrowser.dir/kdevcontextbrowser_autogen/mocs_compilation.cpp.o
[ 63%] Building CXX object plugins/contextbrowser/CMakeFiles/kdevcontextbrowser.dir/contextbrowser.cpp.o
[ 63%] Building CXX object plugins/contextbrowser/CMakeFiles/kdevcontextbrowser.dir/contextbrowserview.cpp.o
ERROR: Job failed: execution took longer than 1h0m0s seconds