Prevent dropping distribution keys for MODULO

Prevent dropping distribution keys for MODULO

Due to a coding issue in IsDistColumnForRelId() it was possible to drop
columns that were used as modulo distribution keys. A simple example
demonstrates the behavior:

    CREATE TABLE t (a INT, b INT, c INT) DISTRIBUTE BY MODULO (a);
    ALTER TABLE t DROP COLUMN a;

    test=# \d+ t
                              Table "public.t"
     Column |  Type   | Modifiers | Storage | Stats target | Description
    --------+---------+-----------+---------+--------------+-------------
     b      | integer |           | plain   |              |
     c      | integer |           | plain   |              |
    Distribute By: MODULO(........pg.dropped.1........)
    Location Nodes: ALL DATANODES

With this commit, the ALTER TABLE command fails as expected:

    ERROR:  Distribution column cannot be dropped

The commit simplifies the coding a bit, and removes several functions
that were not needed anymore (and unused outside locator.c).