lib: add meta_hash to connection, eliminate hash_offt

With a meta_hash at each connection (similar to easy handle, let
multi_ev.c store its pollsets as meta data, no longer needing its own
hashes.

This eliminates the last use of Curl_hash_offt. Remove it.

Closes #17095

docs/internals/HASH.md

@@ -149,40 +149,3 @@ Called repeatedly, it iterates over all the entries in the hash table.
 
 Note: it only guarantees functionality if the hash table remains untouched
 during its iteration.
-
-# `curl_off_t` dedicated hash functions
-
-## `Curl_hash_offt_init`
-
-~~~c
-void Curl_hash_offt_init(struct Curl_hash *h,
-                         size_t slots,
-                         Curl_hash_dtor dtor);
-~~~
-
-Initializes a hash table for `curl_off_t` values. Pass in desired number of
-`slots` and `dtor` function.
-
-## `Curl_hash_offt_set`
-
-~~~c
-void *Curl_hash_offt_set(struct Curl_hash *h, curl_off_t id, void *elem);
-~~~
-
-Associate a custom `elem` pointer with the given `id`.
-
-## `Curl_hash_offt_remove`
-
-~~~c
-int Curl_hash_offt_remove(struct Curl_hash *h, curl_off_t id);
-~~~
-
-Remove the `id` from the hash.
-
-## `Curl_hash_offt_get`
-
-~~~c
-void *Curl_hash_offt_get(struct Curl_hash *h, curl_off_t id);
-~~~
-
-Get the pointer associated with the specified `id`.

docs/internals/UINT_SETS.md

@@ -126,6 +126,3 @@ Iterating a sparse bitset works the same as for bitset and table.
 At last, there are places in libcurl such as the HTTP/2 and HTTP/3 protocol implementations that need
 to store their own data related to a transfer. `uint_hash` allows then to associate an unsigned int,
 e.g. the transfer's `mid`, to their own data.
-
-This is just a variation of `hash_offt` that can associate data with a `connection_id`. Which
-is a specialization of the generic `Curl_hash`.