[toplevel] Respect COQ_COLORS environment variable

Since 3fc02bb2034a ("[pp] Move terminal-specific tagging to the
toplevel."), the COQ_COLORS environment variable has been ignored,
since init_terminal_output unconditionally called init_tag_map with
the default colors, overwriting any custom colors that had been
previously set. Fix this by creating a separate function,
default_styles, to set the default colors.

Also, remove the clear_styles function, as it was only called in one
place and did nothing (since tag_map is empty to begin with).

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