Remove close_rfcomm_instead_of_reset flag

The flag has been enabled by default since Android 24Q2, and there have been no reports of any problems. Therefore, it is safe to remove the flag and make the behavior permanent.

Bug: 347316578
Change-Id: Ie83e19d177264c65450c3b4673b74fa977399454
Flag: com.android.bluetooth.flags.close_rfcomm_instead_of_reset
Test: m packages/modules/Bluetooth