Revert "build: Require Meson >= 0.50"

This reverts commit 17777040.

Some projects that include Tracker haven't yet updated to Meson 0.50. This
change broke gnome-build-meta for example:

The install: arg isn't actually required as for configure_file() targets
Meson will infer whether an install is needed by the presence of the
install_dir: arg.
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