refactor(bigquery): remove unused connect function in ibis/__init__.py

[cpcloud]

Seems like this is unused. We go through the backend via a module-level __getattr__ now, and have been for a while.

[cpcloud]

BigQuery tests are passing with this PR:

…/ibis on  remove-bigquery-connect is 📦 v10.5.0 via 🐍 v3.13.3 via ❄️  impure (ibis-3.13-env) took 6s
❯ pytest -m bigquery -n auto --dist loadgroup -q -x
bringing up nodes...
.................xxsssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss............s....xx.....x......s...................................s.x.. [  8%]
........x........xx.x..xsssssssssssssssssssss..xx..x.x.xxx...xx.x.x...x........xx........xx.xx.....x........x.x.......x......x............x...x..x.......x...x......x..x.............x.x...... [ 16%]
............x.....x......x....x.............x...xx............x.....x.......................x.x.......................x.....x.....................x.....x................x.x........x......... [ 24%]
.x..............x.........................x..x..x.......x................x....x...xx.......x....x.x.......x......x....x...................x.....x......x.........s...x........x..s.x.......... [ 32%]
......x..s.s.xx.....x.x...........x................xx.........x...................x........x.....x........x..........x.......x...........................x......x..x..xx...........xx......xx. [ 41%]
.x.....x.xx.........x.x...xx......x.....x....x...x..x......x..xx..X....x..xX.....x.x.x.............x.........x.X.x.......x.....x............x.xxx....x..xx.........x....x.Xxx............x.... [ 49%]
.xxxx.x....x.xx..x..xxxxxx.xxxxxx.xxxxx.xx.xxxxxx.xxxxx.xxxxxxxxxxxxxxx.x.xxx.xxxxxxxxxxxxxxxxxx.xxxxxxxxxxxxxxxxx.xxxxx..xxxxxxxxxxxxxxxxx.x.xxx.x..x.x.x..x...................s.x.x..x..x... [ 57%]
x.x...x......x....xxxxx.......x.xx....xx...x...x...x....x...xx..x....x..x...x......x.....x..........xx...xx...x..x.....x...x..x..xx...x..xx.x..............x..xx...xxxx......xxx........x..x.. [ 65%]
xxx.........x..xx.x...xx...........x..............xx.x..........xx...x..............x...........xx......x.................xx..............xx.................................................. [ 74%]
...x.............................x..................x...................x..................................................................................................................... [ 82%]
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1810 passed, 132 skipped, 358 xfailed, 4 xpassed in 439.61s (0:07:19)

[jeandsantos]

@cpcloud I actually use the ibis.backends.bigquery.connect() function. Would it be possible to have a DeprecationWarning raised next time such functions/methods are removed?