Stop writing /GLOBAL/BField/Maps into in-file metadata

Merged Vakhtang Tsulaia requested to merge tsulaia/athena:master-IOVMetadata-cleanup into master

It has been agreed several times in the past that we should not write COOL folders into in-file metadata. This commit cleans up all python fragments that write /GLOBAL/BField/Maps, and also cleans up the obsolete toMetaData() method of the CondDB python class

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