sm8450-common: Unset BOARD_USERDATAIMAGE_PARTITION_SIZE
This not only saves a lot of build time when building with
TARGET_USERIMAGES_SPARSE_EXT_DISABLED and/or
TARGET_USERIMAGES_SPARSE_F2FS_DISABLED which is required
on A14, but also removes the need for the build system to
temporarily create a 40+GB userdata image.
Which means that less free space is required to start the
build.
Change-Id: Ibf48e7cdf566a4653e85352a79170b354d917234
SGCMarkus