Archaeologists in London have discovered the remains of an Elizabethan theater where some of William Shakespeare’s plays were first performed — a venue immortalized as “this wooden O’’ in the prologue to “Henry V.’’ Experts from the Museum of London said Wednesday they had uncovered part of the gravel yard and gallery walls of the 435-year-old Curtain Theatre in Shoreditch, just east of London’s business district. The remains — of a polygonal structure, typical of 16th-century theaters — were found behind a pub on a site marked for redevelopment.