n_m_remove_indexed_terms: reduce number of Xapian API calls.

Previously this functioned scanned every term attached to a given
Xapian document. It turns out we know how to read only the terms we
need to preserve (and we might have already done so). This commit
replaces many calls to Xapian::Document::remove_term with one call to
::clear_terms, and a (typically much smaller) number of calls to
::add_term. Roughly speaking this is based on the assumption that most
messages have more text than they have tags.

According to the performance test suite, this yields a roughly 40%
speedup on "notmuch reindex '*'"

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