In exploring our current print-range support, I've discovered that we have an ancient partial implementation for restricting our print operation to even/odd pages, but it doesn't actually work (and I don't think it's ever really worked).

Some facts:
(1) We don't expose any UI for this configuration on Windows or Mac, and I don't think we intend to expose UI for it in our new cross-platform "tab-modal print dialog" (not at this point, at least).

(2) We do expose UI for this on Linux (via the system dialog "Page Setup" tab, "Only Print:" dropdown, which has "Even"/"Odd" as options) -- but it doesn't have any effect! I tested the latest Nightly, and various nightlies going back through time back to Nightly 2012-01-01; and I always ended up with all pages being printed. (I used https://www.mozilla.org/en-US/MPL/2.0/ as my testcase, and I chose "Only Print: Even Sheets", with Print To File as my printer.)

(3) We have code for this which is simply backed by prefs (see bug 35228 comment 23); but it doesn't look like there's any UI for reliably configuring those prefs.

We should simply remove the existing non-functional code for the time being -- we can add back actually-working code later on, if/when we decide to support this feature for real.

See https://developer.gnome.org/gtk3/stable/GtkPrintUnixDialog.html for
documentation on the flag in question here (GTK_PRINT_CAPABILITY_PAGE_SET).

After this change, the "Only Print: All Sheets | Even Sheets | Odd Sheets"
dropdown-menu will be grayed out & disabled, which will accurately indicate
that we don't support this feature.

This code seems like it might work, but it seems to have only ever been backed
by per-printer about:config prefs. I believe we only ever exposed UI for this
feature on Linux, via the native GTK dialog; and even there, the UI doesn't
actually seem to have done anything -- it was never wired up to the actual
implementation of even/odd page-skipping.

Depends on D92527

https://hg.mozilla.org/mozilla-central/rev/d67cb6bf0152
https://hg.mozilla.org/mozilla-central/rev/f8088b3273af