Remove unnecessary autoconf check for pkexec --disable-internal-agent option (#776437)

The pkexec --disable-internal-agent option was introduced by this commit
[1] in polkit 0.98 [2] which is before the minimum 0.102 version needed
to run X11 apps.  Therefore the option will always be available.  Remove
the unnecessary check.

[1] https://cgit.freedesktop.org/polkit/commit/?id=f071d4561dfe8dd9cfd4e29fddec7bc82fd658aa
    pkexec: add --disable-internal-agent option

[2] https://cgit.freedesktop.org/polkit/tree/NEWS?id=0.98#n25
    polkit NEWS entry for 0.98

Bug 776437 - GParted fails to run as root under Wayland