Remove unused lines to fix dialyzer

[varnerac]

Fixes the following dialyzer errors:

bson-erlang % rebar3 dialyzer
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling bson
===> Dialyzer starting, this may take a while...
===> Add debug_info to compiler options (erl_opts) if Dialyzer fails to load Core Erlang.
===> Updating plt...
===> Resolving files...
===> Checking 202 files in _build/default/rebar3_23.3.4.1_plt...
===> Doing success typing analysis...
===> Resolving files...
===> Analyzing 3 files with _build/default/rebar3_23.3.4.1_plt...

src/bson.erl
Line 312: Guard test is_atom(Key::bitstring()) can never succeed
Line 313: Guard test is_integer(Key::bitstring()) can never succeed
Line 314: Guard test is_float(Key::bitstring()) can never succeed
Line 315: Guard test is_list(Key::bitstring()) can never succeed
===> Warnings written to _build/default/23.3.4.1.dialyzer_warnings
===> Warnings occurred running dialyzer: 4