[2pt] Delete documentation entry for net.box conn:timeout()

[TarantoolBot]

Discuss what we do with deprecated sections in the documentation.

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#conn-timeout
SME: @ locker
Peer reviewer: @

#Details
conn:timeout() was deprecated more than four years ago (in 1.7.4). Since
it results in performance degradation in case timeout is not specified
explicitly in request options, we decided to drop it. Please remove it
from the documentation as well:

https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#conn-timeout
Requested by @locker in tarantool/tarantool@ad82655.