Remove (nearly-)unused platforms

[toy]

Following an experiment with fake platform gem, per platform downloads table show that downloads of freebsd and openbsd platform gems are very low (rarely going over fake platform gem). Supporting multiple platforms makes it harder to do anything beside maintenance, so support of freebsd and openbsd is going to be removed.

+----------------+------+----------+--------+--------+------+---------+---------+-------+---------+---------+---------+-------+
| version        | days | base dls | darwin | darwin | fake | freebsd | freebsd | linux | linux   | openbsd | openbsd | ruby  |
|                |      |          | x86    | x86_64 | x42  | x86     | amd64   | x86   | x86_64  | x86     | x86_64  |       |
+----------------+------+----------+--------+--------+------+---------+---------+-------+---------+---------+---------+-------+
| 0.5.6.20190728 |   18 |      160 |     +4 |  +1455 |   +6 |      +4 |       = |    +6 |   +3419 |      +4 |      +2 |  +320 |
| 0.5.6          |   10 |      160 |    +25 |   +734 |   +2 |       = |       = |    +2 |   +2076 |      +2 |      +2 |  +281 |
| 0.5.5          |    4 |      166 |     +1 |   +779 |   +1 |       = |       = |    +5 |   +1340 |      +1 |      +3 |  +150 |
+----------------+------+----------+--------+--------+------+---------+---------+-------+---------+---------+---------+-------+