base: Remove bunk default impl of (>>=)

Without 'join' as a part of Monad, this default implementation is bogus,
and is a leftover from d94de87252d0fe2ae9.

Signed-off-by: Austin Seipp <austin@well-typed.com>
base: Add some notes about the default impl of '(>>)'

See Note [Recursive bindings for Applicative/Monad]. This documents the
tricky little details that kept me occupied for so long with this patch,
and why exactly we deviate from the original proposal.

Signed-off-by: Austin Seipp <austin@well-typed.com>