[M67 Migration] Prevent double free of MailboxManager unique pointer

[M67 Migration] Prevent double free of MailboxManager unique pointer

Crash is observed at browser shutdown as MaiboxManager is getting
deleted twice, once when GpuChannelManager is destroyed and again
in SharedMailboxManager. Since we already maintain a unique pointer
(static) for MailboxManager in SharedMailboxManager, GpuChannelManager
doesn't necessarily need to have another.

This patch also avoids creation of GLShareGroup and MailboxManager
in GpuChannelManager as they are anyway overriden in GpuChildThreadEfl.

Reference :
https://review.tizen.org/gerrit/#/c/179612

Change-Id: I3d4e7e211bb1260ccf28a03da92a94b39f6a16d0
Signed-off-by: Uzair <uzair.jaleel@samsung.com>