Remove the SIP to PV conversion

In the split_exp package we are converting the WCS PV keywords to SIP to read the WCS with astropy, and this is not necessary. At the moment astropy cannot read the current WCS because the TYPE of the WCS is set to RA--TAN instead of RA--TPV. This was not a problem in the past because astropy allows inconsistency between the TYPE and the provided keywords, but now it is more strict about it. Also the TAN and TPV conventions are the same. I would propose to remove the conversion in the pipeline and instead update the TYPE in the header to avoid the dependence to an external package and changing the original WCS.