Remove stsci.tools.gfit dependency in mktrace

[sean-lockwood]

stsci.tools deprecation warnings:

.../stsci/tools/nmpfit.py:8: UserWarning: NMPFIT is deprecated - stsci.tools v 3.5 is the last version to contain it.
  warnings.warn("NMPFIT is deprecated - stsci.tools v 3.5 is the last version to contain it.")

.../stsci/tools/gfit.py:18: UserWarning: GFIT is deprecated - stsci.tools v 3.4.12 is the last version to contain it.Use astropy.modeling instead.
  warnings.warn("GFIT is deprecated - stsci.tools v 3.4.12 is the last version to contain it."

gfit shows up in mktrace:
https://github.com/spacetelescope/stistools/blob/master/stistools/mktrace.py#L8