mmc: core: Drop unused define

The last user of MMC_OPS_TIMEOUT_MS was recently removed, however the
define stayed around. Let's remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Baolin Wang <baolin.wang7@gmail.com>
Tested-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200204085449.32585-3-ulf.hansson@linaro.org