Doc: remove claim that all \pset format options are unique in 1 letter.

This hasn't been correct since 9.3 added "latex-longtable".

I left the phraseology "Unique abbreviations are allowed" alone.
It's correct as far as it goes, and we are studiously refraining
from specifying exactly what happens if you give a non-unique
abbreviation.  (The answer in the back branches is "you get a
backwards-compatible choice", and the answer in HEAD will shortly
be "you get an error", but there seems no need to mention such
details here.)

Daniel Vérité

Discussion: https://postgr.es/m/cb7e1caf-3ea6-450d-af28-f524903a030c@manitou-mail.org

Author: tglsfdc

doc/src/sgml/ref/psql-ref.sgml

@@ -2589,8 +2589,7 @@ lo_import 152801
           <literal>latex</literal> (uses <literal>tabular</literal>),
           <literal>latex-longtable</literal>, <literal>troff-ms</literal>,
           <literal>unaligned</literal>, or <literal>wrapped</literal>.
-          Unique abbreviations are allowed.  (That would mean one letter
-          is enough.)
+          Unique abbreviations are allowed.
           </para>
 
           <para><literal>unaligned</literal> format writes all columns of a row on one