cameraMapper _standardizeExposure should not try to create a WCS for each amp

After DM-20154, updates were made to _standardizeExposure which now creates a WCS from visit metadata. The problem is that _standardizeExposure is called per amp when reading raws, which leads to a cascade of warnings with obs_lsst because there are a lot of amps, and the visitInfo hasn't been created yet with the appropriate metadata to create a WCS. However, even if the warnings aren't there, it is unnecessary and inefficient to create a WCS for each amp, when this should be done once at the ccd-level after assembly (and after the appropriate metadata has been created).

I believe this should be a quick fix that simply checks if it is called with the amp level before trying to set the wcs. At the same time maybe the per-amp setting of the filter is also unnecessary?