Remove outdated comments from the regression test files.

Since 2004, int2 and int4 operators do detect overflow; this was fixed by
commit 4171bb86.

Extracted from a larger patch by Andres Freund.

src/test/regress/expected/int2.out

 --
 -- INT2
--- NOTE: int2 operators never check for over/underflow!
--- Some of these answers are consequently numerically incorrect.
 --
 CREATE TABLE INT2_TBL(f1 int2);
 INSERT INTO INT2_TBL(f1) VALUES ('0   ');

src/test/regress/expected/int4.out

 --
 -- INT4
--- WARNING: int4 operators never check for over/underflow!
--- Some of these answers are consequently numerically incorrect.
 --
 CREATE TABLE INT4_TBL(f1 int4);
 INSERT INTO INT4_TBL(f1) VALUES ('   0  ');

src/test/regress/sql/int2.sql

 --
 -- INT2
--- NOTE: int2 operators never check for over/underflow!
--- Some of these answers are consequently numerically incorrect.
 --
 
 CREATE TABLE INT2_TBL(f1 int2);

src/test/regress/sql/int4.sql

 --
 -- INT4
--- WARNING: int4 operators never check for over/underflow!
--- Some of these answers are consequently numerically incorrect.
 --
 
 CREATE TABLE INT4_TBL(f1 int4);