meson.build: drop explicit install:true from configure_file
meson implicitly sets install to whether install_dir is nonzero. Which means it's superfluous anyway and removing it drops the meson warning:
WARNING: Project specifies a minimum meson_version '>= 0.41.0' but uses features which were added in newer versions:
- 0.50.0: {'install arg in configure_file'}
Fixes #334 (closed)