http2: remove unused addConn

[jaricftw]

addConn is dead code that's not used any more.

addConnLocked is used when a new connection
needs to be added to the connection pool, and the callers
always manage the lock.

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[jaricftw]

@googlebot I signed it!

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Message from Brad Fitzpatrick:

Patch Set 1: Run-TryBot+1 Code-Review+2

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This PR is being closed because golang.org/cl/230201 has been merged.