fix: page_number appears in partition_html metadata if include_metadata=False

[Coniferish]

Summary

Fixes bug from #592
Creates test for partitioning html from filename when metadata=False
Creates _remove_element_metadata method

Testing

from unstructured.partition.html import partition_html

filename = "example-docs/example-10k.html"
elements = partition_html(filename=filename, include_metadata=False)
elements[-1].metadata

[MthwRobinson]

Implementation looks good. Just one small question. Could you also add a CHANGELOG entry?

[MthwRobinson]

LGTM - thanks for the contribution!