CPack/NSIS: Fix uninstall command when run from installer

The quoting introduced by commit eb3b3bacdc (CPack/NSIS: Fix uninstall
on Windows using "Apps & Features", 2021-09-13, v3.22.0-rc1~136^2)
created two errors in the uninstaller call: double quoting of the
uninstaller executable, and quotes added to the `_?=` argument which
does not support them.  Simplify the command.
1 file changed