[allocator.requirements] Removed unused variable a3 in Table 27.

Author: jacobsa

source/lib-intro.tex

@@ -1643,7 +1643,6 @@
 \tcode{YY}      &   the type \tcode{allocator_traits<Y>}    \\ \rowsep
 \tcode{t}       &   a value of type \tcode{const T\&}   \\ \rowsep
 \tcode{a, a1, a2}   &   values of type \tcode{X\&}      \\ \rowsep
-\tcode{a3}      &   an rvalue of type \tcode{X}      \\ \rowsep
 \tcode{b}       &   a value of type \tcode{Y}           \\ \rowsep
 \tcode{c}       &   a pointer of type \tcode{C*} through which indirection is valid \\ \rowsep
 \tcode{p}       &   a value of type \tcode{XX::pointer}, obtained