mmc: core: Enable erase/discard/trim support for all mmc hosts

Step by step, mmc host drivers and the mmc core have been improved in
regards to support erase/discard/trim operations. We have now reached a
point when it no longer seems reasonable to use an opt-in approach to
enable the functionality. Therefore, let's switch to make the operations
always supported.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200508112853.23525-1-ulf.hansson@linaro.org
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>