defconfig: mdmcalifornium: Remove CONFIG_ARCH_MDM9607 flag

This flag is added for MDM9607 target which is supposed to use its own
defconfig file. Hence remove it to avoid breaking WLAN for mdmcalifornium
target.

Change-Id: I849f988e5ef7226b0066deda27041ada8e15122b
Signed-off-by: Yue Ma <yuem@codeaurora.org>