cleanup clobber/mode

[Carreau]

It looks like there is a redundancy between a clobber/mode keyword arguments that can be simplified; my guess is it used to be a boolean, and has been changed for mode that allow 3 states (r,w,a):

$ rg clobber
zarr/convenience.py
75:    clobber = mode == 'w'
78:    store = normalize_store_arg(store, clobber=clobber,
135:    store = normalize_store_arg(store, clobber=True)
206:    store = normalize_store_arg(store, clobber=True)

zarr/hierarchy.py
1038:def _normalize_store_arg(store, *, clobber=False, storage_options=None, mode=None):
1041:    return normalize_store_arg(store, clobber=clobber,
1151:    clobber = mode != "r"
1153:        store, clobber=clobber, storage_options=storage_options, mode=mode
1156:        chunk_store = _normalize_store_arg(chunk_store, clobber=clobber,

zarr/storage.py
1398:        # get clobbered
1701:            flag = 'c' + flag[1:]  # don't clobber an existing database
1721:                        flag = 'c' + flag[1:]  # don't clobber an existing database

zarr/creation.py
132:def normalize_store_arg(store, clobber=False, storage_options=None, mode='w'):
136:        mode = mode if clobber else "r"
453:    clobber = (mode == 'w')
454:    store = normalize_store_arg(store, clobber=clobber, storage_options=storage_options, mode=mode)
456:        chunk_store = normalize_store_arg(chunk_store, clobber=clobber,

Clobber also does not seem to be exposed to any external API so can likely be removed.