[lex.string] Remove unused term from the index

While 'character string' is indexed for this block of text,
there is no definition for this term present.  Searching the
standard for this term, outside a 5 or 6 uses in library
clauses, the only other matches are as prefixes for the
larger term of power, 'character string literal'.
As this term appears to be unused, and has no definition,
let's remove it from the index.

Author: AlisdairM

source/lex.tex

@@ -1851,7 +1851,6 @@
 
 \pnum
 \indextext{literal!string}%
-\indextext{character string}%
 \indextext{string!type of}%
 \indextext{type!\idxcode{wchar_t}}%
 \indextext{prefix!\idxcode{L}}%