defconfig: sdm845: remove CONFIG_EXTCON

Commit <083d5e24> ("mmc: add hot plug support using extcon framework")
adds CONFIG_EXTCON as default option when MMC driver is enabled so this
change removes explicit enablement of CONFIG_EXTCON from sdm845_defconfig.

Change-Id: Ice12370bd375b883258e2a69229d6685f1ca2461
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>