macos: Remove SDKROOT from AklogAuthPlugin project

macos: Remove SDKROOT from AklogAuthPlugin project

An attempt to build AklogAuthPlugin with the default SDK fails:

  xcodebuild --project AklogAuthPlugin.xcodeproj

  error: unable to find sdk 'macosx10.6' (in target 'AklogAuthPlugin'
         from project 'AklogAuthPlugin')
  warning: There are no architectures to compile for because the
         VALID_ARCHS build setting is an empty list. (in target
         'AklogAuthPlugin' from project 'AklogAuthPlugin')
  ** BUILD FAILED **

Remove the SDKROOT=macosx10.6 specification from the project file.

Change-Id: Iecec6d6c56bcb3d942bd4ed811e61fe4ec016b1e
Reviewed-on: https://gerrit.openafs.org/16151
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>