Deprecate enable-private-internal-component-mirror

:Release Notes:
Specifying --enable-private-internal-component-mirror on the mcf
command line no longer has any effect.

:Detailed Notes:
When this command line option is used, the fetch URL for all github
hosted repositories is changed to reference an internal mirror. Sadly,
this invalidates shared state for each of those components, forcing local
builds to take considerably longer than necessary while they fetch and
rebuild components such as WebKit.

Eventually, this option will be completely removed from mcf. This commit
simply changes mcf to ignore the option if specified.

:Testing Performed:
Test builds to prove that shared state is now used correctly.

:QA Notes:

:Issues Addressed:
[GF-4227] Drop the --enable-private-internal-component-mirror/
          --disable-private-internal-component-mirror options to mcf

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>

Change-Id: I692d1d438f9bf1fff26933eac847dd703a7cdbab
Reviewed-on: https://g2g.palm.com/3888
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>