BUG/MINOR: backend: do not set sni on connection reuse

BUG/MINOR: backend: do not set sni on connection reuse

When reusing a backend connection, do not reapply the SNI on the
connection. It should already be defined when the connection was
instantiated on a previous connect_server invocation. As the SNI is a
parameter used to select a connection, only connection with same value
can be reused.

The impact of this bug is unknown and may be null. No memory leak has
been reported by valgrind. So this is more a cleaning fix.

This commit relies on the SF_SRV_REUSED flag and thus depends on the
following fix :
  BUG/MINOR: backend: restore the SF_SRV_REUSED flag original purpose

This should be backported up to 2.4.

(cherry picked from commit 655dec81bde67a3e1acb146b18e6a038fe8c797b)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4568a4bba6f7e6f19a9f1d184ecf6e572b6942f1)
[wt: adjusted to fit with the existing condition]
Signed-off-by: Willy Tarreau <w@1wt.eu>