mmc: core: Remove unused variable cached_ext_csd

There is one variable defined in mmc_card, which
is not being used anywhere. Also it is wrongly
being freed from mmc_remove_card and causing
"double kfree" related issues.
To handle this, remove unused cached_ext_csd variable.

Change-Id: I75f376ec787a46a2a16c38f88f444bd8ff9c86c4
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>