Fix build-time warning with meson 0.50.1

Commit 6666dee5 ("man: Add weston-bindings(7) describing
desktop shell shortcuts") adds in file "man/meson.build" the line
        install: true,
This line triggers a warning with meson 0.50.1:
        WARNING: Project targetting '>= 0.47' but tried to use
        feature introduced in '0.50.0': install arg in configure_file

Accordingly with
        https://github.com/mesonbuild/meson/issues/5048


the line was silently ignored by meson before 0.50.0

One possible fix for this warning would require updating the
minumum version of meson required by weston, but then forcing every
builder to update meson.
Instead, since all the other instances in "man/meson.build" of
configure_file don't use the feature "install:", it seams safe to
simply remove the feature for the instal of "weston-bindings.man".

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>