[B] Corrected solid property of various materials. Fixes BUKKIT-5671, BUKKIT-5559

**The Issue:**
Various blocks are marked as solid blocks whilst the player may pass freely through them. Likewise, `FLOWER_POT` was not recognized as a solid block.

**Justification for this PR:**
Previously, `WALL_SIGN` was recognised as a solid block although players can pass through this block.

Likewise, `WOOD_PLATE`, `STONE_PLATE`, `GOLD_PLATE` and `IRON_PLATE` were treated as solid blocks although none of those blocks prevent the player from passing through.

Lastly, `FLOWER_POT` was not recognised as a solid block. This block does prevent the player from passing through.

**PR Breakdown:**
This PR corrects the `isSolid()` method in Material.java, removing all materials mentioned above and adding `FLOWER_POT` as a recognised solid block.

**Testing Results and Materials:**
The changes in this PR are too trivial to require individual testing. The changes in this PR compile without compile warnings nor compile errors.

**Relevant PR(s):**
Bukkit/Bukkit#1087 - The original code was inspired from @xa112. His PR did not qualify to the contributing guidelines.

**JIRA Tickets:**
https://bukkit.atlassian.net/browse/BUKKIT-5671
https://bukkit.atlassian.net/browse/BUKKIT-5559