Cleanup of rmsimage.py

[AlexKurek]

• max_isl_brightsize is never used.
• mylog = mylogger.logging.getLogger("PyBDSF."+img.log+"Rmsimage.Calcmaps ") was a duplicate - the same line is 2 lines above.
• thratio, avg_max_isl_size, maxrms, maxmean, pt_src_scale are computed but not used.

[AlexKurek]

• after removing the above variables also max_isl_size_lowthresh, bright_threshold, isl_snr, mask_small and isl_size_lowthresh are not needed
• from . import _cbdsm is not used
• from .image import Image, NArray, List is not used
• moved floor, ceil import tothe top of the file

Changing the name of the PR, since its not longer so minor.