boards: silabs: Remove redundant enabling of BT_SEND_ECC_EMULATION

This option gets the default value "y" whenever HAS_BT_CTLR is enabled, and
since the EFR32 HCI driver now selects HAS_BT_CTLR the right thing happens
either way.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>