Unneeded cython *imports in dist_metrics.pxd

[jakirkham]

It looks like cython is both cimported and imported in dist_metrics.pxd (only the former is needed with cython unless pure Python mode is used)

hdbscan/hdbscan/dist_metrics.pxd

Lines 6 to 7 in 2e7112d

	import cython
	cimport cython

However looking at the module it doesn't appear the cython namespace is used anywhere. So maybe both of these can be dropped?