Revert "FIX: replace spurious 1.0E+12 by finfo(float).max in slsqp"

This reverts commit ccecd81bc4490b1cf71be85d31d838013d8fca7e.

Closes #3108.
DOC: Document how SLSP handles infinite values in bounds
TST: Add a test for SLSQP with bounds
Merge pull request #3110 from dlax/bug/slsqp-bounds

BUG: optimize: restore previous behavior with infinite bounds in SLSQP, document, add tests