There always was a one-to-one mapping between dict_tree_t and dict_index_t.

This saves 6 machine words per B-tree index in the data dictionary
cache plus the memory allocation overhead.  We save one mem_heap_t
object per index (15 machine words).  Considering the internal
fragmentation of the buddy allocator in mem_area_alloc(), this should
save 32 machine words per index (128 bytes on 32-bit systems and 256
bytes on 64-bit systems).  (Bug #20877)

struct dict_tree_struct, dict_tree_t: Remove.

struct dict_index_struct: Add page and lock.

dict_tree_create(): Remove.  Replace the invocation with
assignment to index->page and a call to rw_lock_create(&index->lock).

dict_tree_free(): Remove.  Replace the invocation wtih
a call to rw_lock_free(&index->lock).

dict_index_get_tree(): Remove.

dict_tree_get_space_reserve(): Rename to dict_index_get_space_reserve()
and remove the parameter, which was unused.

btr_level_list_remove(): Remove the unused parameter "tree".

Replace the occurrences of "tree" with "index" in names of variables,
functions and data types, e.g. "dict_tree_t tree" becomes
"dict_index_t index".  Remove local variables "tree" or "index" of
functions that needed both "tree" and "index".