Remove all traces of deprecated QgsFieldMap.

Some parts of the code still used the old QgsFieldMap, in particular the MSSQL provider code. This caused a crash when importing a vector layer into an MSSQL DB, since

    createEmptyLayer_t * pCreateEmpty = ( createEmptyLayer_t * ) cast_to_fptr( myLib->resolve( "createEmptyLayer" ) );

in `QgsVectorLayerImport::QgsVectorLayerImport` reinterpret-casts the `QgsMssqlProvider::createEmptyLayer` function pointer to a function pointer with incompatible signature, resulting in a `QgsFields` being used as a `QgsFieldMap`.

Funded by Sourcepole.