[go-migration] Remove obsolete struct from JRE providers

[kiril-keranov]

This Context struct is the same like the one in common/context.go but probably some leftover as it is not used anywhere. The one in common/context.go is the one broadly used around in the container, framework and jre logic.
Also removing the declared but not used labelMatch variable for the postgresql framework, as the check based on it was removed with the previous change.

[kiril-keranov]

@ramonskie I guess this should be merged soon in order to prevent other testers to stumble on the BuildpackCompileFailed - App staging failed in the buildpack compile phase when executing staging from the go-migration branch.