dict_sys->col_hash, and DICT_POOL_PER_COL_HASH.  (Bug #20877)

The col->table pointer was only needed for maintaining a hash table of
all defined columns in all tables.  The hash table was only looked up
in dict_index_find_cols().  Removing the col->hash and col->table
pointers reduces the size of a table column by two machine words
(usually 8 or 16 bytes).

dict_col_add_to_cache(), dict_col_reposition_in_cache(),
dict_col_remove_from_cache(): Remove.  These only updated dict_sys->col_hash.

dict_index_find_cols(): Use a linear search instead of the hash table.
The time complexity is affected, but this function is only invoked by
dict_index_add_to_cache(), and we only search the columns of a single
table (typically at most a few dozen) as opposed to all columns of all
tables.