Merge battle graphics into battle_terrain and remove un-included files

The previous battle folder contained the used (and until recently undocumented) battle terrain graphics, while everything in the previous battle_terrain folder contained unused data (which is now at battle_terrain/unused, minus the files that never get included in the rom).

graphics/battle_terrain/underwater/palette.pal

@@ -1,51 +0,0 @@
-JASC-PAL
-0100
-48
-0 0 0
-197 230 255
-131 139 197
-148 197 246
-172 205 255
-131 164 213
-189 213 255
-148 180 222
-205 222 255
-197 230 255
-0 0 0
-148 164 238
-156 189 238
-172 205 238
-189 213 246
-197 222 255
-0 0 0
-197 230 255
-131 139 197
-148 197 246
-172 205 255
-131 164 213
-189 213 255
-148 180 222
-205 222 255
-197 230 255
-0 0 0
-148 164 238
-156 189 238
-172 205 238
-189 213 246
-197 222 255
-0 0 0
-98 164 230
-131 197 255
-172 238 255
-255 255 255
-205 255 255
-0 0 0
-0 0 0
-0 0 0
-0 0 0
-0 0 0
-0 0 0
-98 98 172
-131 131 230
-172 172 255
-255 255 255