Remove hstore's text => text operator.

Since PostgreSQL 9.0, we've emitted a warning message when an operator
named => is created, because the SQL standard now reserves that token
for another use.  But we've also shipped such an operator with hstore.
Use of the function hstore(text, text) has been recommended in
preference to =>(text, text).  Per discussion, it's now time to take
the next step and stop shipping the operator.  This will allow us to
prohibit the use of => as an operator name in a future release if and
when we wish to support the SQL standard use of this token.

The release notes should mention this incompatibility.

Patch by me, reviewed by David Wheeler, Dimitri Fontaine and Tom Lane.