Converts the D-glycero-D-manno-heptose 1,7-bisphosphate intermediate into D-glycero-D-manno-heptose 1-phosphate by removing the phosphate group at the C-7 position. D-glycero-D-manno-heptose 1,7-bisphosphate + H2O = D-glycero-D-manno-heptose 1-phosphate + phosphate Monomer. Belongs to the GmhB family.