Remove btree_gist's useless logic for encoding-aware truncation.

gbt_var_node_cp_len() contained logic to ensure that its choice of
a common prefix length didn't truncate away part of a multibyte
character.  However, that was really dead code, because we have not
allowed truncation of text-string data types since ef770cbb6, and
it seems unlikely that that behavior could ever get resurrected.
The code is still reachable via gbt_var_penalty, but for that
usage it hardly matters if we break in the middle of a multibyte
character: we're just calculating a small correction factor that
is arguably bunkum anyway in non-C locales.

Hence, delete said code.  That actually removes all need for
gbtree_vinfo.eml, which allows const-ification of the gbtree_vinfo
structs in which we were changing it, which removes one headache
for future attempts to thread-ify the backend.

(Curiously, all this infrastructure was itself added by ef770cbb6.
Not sure why Teodor didn't see the contradiction.)

Author: Tom Lane <[email protected]>
Reviewed-by: Ayush Tiwari <[email protected]>
Discussion: 
https://postgr.es/m/AH*avqcykhqgvvpwi1giu4oy.8.1781609375063.hmail.3020001...@tju.edu.cn

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b82d69abf64fc0c2fc6fdd491d7cecb8244680c2

Modified Files
--------------
contrib/btree_gist/btree_bit.c       |  1 -
contrib/btree_gist/btree_bytea.c     |  1 -
contrib/btree_gist/btree_numeric.c   |  1 -
contrib/btree_gist/btree_text.c      | 21 ++------------------
contrib/btree_gist/btree_utils_var.c | 37 ++++++------------------------------
contrib/btree_gist/btree_utils_var.h |  2 --
6 files changed, 8 insertions(+), 55 deletions(-)

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