Fix metadata memory leak

Metadata is not properly removed from MetadataStoreBase after the metadata subject has died/been garbage collected. This results in a memory leak of metadata values.

Test plugin: http://i.devin.codes/bgwS (source: https://gist.github.com/forairan/6e0e36be6c95632d7a85)
I used the "/metatest 1" command several times to check if the metadata was being properly removed.

Output with standard CraftBukkit/Spigot: http://i.devin.codes/M5Ry
Output with patched Spigot: http://i.devin.codes/mXGv