Prevent static plugin triggering autogen dependency on reconfigure

A call to file(WRITE) will unconditionally update the file's timestamp
even if the file's contents don't change. The *Plugin.cpp file was
being written using configure_file() which avoids that, but the .cpp.in
file it was configuring from was being written out using file(WRITE)
every time CMake ran. Autogen saw that file as a dependency and then
regenerated the mocs_compilation.cpp file, which in turn results in
unnecessary rebuilds and relinking when nothing is actually changing.

The file(WRITE) - configure_file() dance is no longer needed anyway,
since the generated *Plugin.cpp file is very simple with no
substitutions required. Therefore, we can simplify that file's
generation with a single file(WRITE) that only executes if the file
contents will change or the file is missing.

Pick-to: 6.1 6.0
Change-Id: I2b7d1ff678b85ea7811969d656555592c9b6865f
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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