Don't update the MTU across multiple A2DP connections that are open

Remove function bta_av_chk_mtu() that updates the MTU to the smallest
value across all open A2DP connections, because that function is not needed.
Such MTU update is problematic for Multi-A2DP, because some codecs cannot
be initialized/used if the MTU is too small. E.g., for LDAC the
MTU value used for the codec initialization must be at least 679.

Bug: 79675742
Test: Manual - (1)Connect Bose QC35; (2)Connect Sony SRS-XB2; (3)Play music
Change-Id: I8048a1bf6df6d4d6291bf3570a95b5182af15b6a