Bluetooth string: removed id="sender" in bt_sm_2_1_

As there is no value assignment for string id="sender" in string bt_sm_2_1,
%1$s will show as garbled char for the user while the capacity of the device is full

bug: b/153128298

Signed-off-by: Yinchu Chen <chenyc5@motorola.com>

Change-Id: Iec7da3fa32b2133263d18bbe3a2f187aa7be99f0