Figure.grdimage: Remove deprecated parameter 'bit_color', use 'bitcolor' instead (deprecated since v0.8.0)

[michaelgrund]

Description of proposed changes

This PR removes the deprecated bit_color parameter from Figure.grdimage (deprecated since v0.8.0).

Related to #3033

Reminders

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• Write detailed docstrings for all functions/methods.
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[seisman]

Prefer to a PR title like:

Figure.grdimage: Remove deprecated parameter 'bit_color', use 'bitcolor' instead (deprecated since v0.8.0)

[seisman]

We had a typo at

pygmt/doc/changes.md

Line 142 in 5098af0

* Figure.image: Deprecate parameter "bit_color" to "bitcolor" (remove in v0.12.0) ([#2635](https://github.com/GenericMappingTools/pygmt/pull/2635))

Please change Figure.image to Figure.grdimage in this file.