BUG #18993: [BUG] Unreachable code in pg_next_dst_boundary()

The following bug has been logged on the website:

Bug reference:      18993
Logged by:          Eugeny Goryachev
Email address:      gorcom2012@gmail.com
PostgreSQL version: 17.4
Operating system:   Ubuntu
Description:

Hello,
In pg_next_dst_boundary() (src/timezone/localtime.c), the following
condition can never be true:
```
if (tcycles - icycles >= 1 || icycles - tcycles >= 1)
    return -1;
```
This is unreachable because icycles is assigned directly from tcycles.

Suggested fix:
Remove this condition entirely

The change is safe as it only removes dead code. All timezone transition
cases are already properly handled by other paths.

Best regards, Eugeny Goryachev