[libc++] Remove P1908 from the status page

The paper doesn't include anything affecting the library. AFAICT there isn't even anything to do for the compiler, since it just reserved the std attribute namespace.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D138128

libcxx/docs/Status/Cxx20Papers.csv

@@ -171,7 +171,6 @@
 "`P1739R4 <https://wg21.link/P1739R4>`__","LWG","Avoid template bloat for safe_ranges in combination with ""subrange-y"" view adaptors","Prague","|Complete|","15.0"
 "`P1831R1 <https://wg21.link/P1831R1>`__","LWG","Deprecating volatile: library","Prague","* *",""
 "`P1868R2 <https://wg21.link/P1868R2>`__","LWG","width: clarifying units of width and precision in std::format","Prague","|Complete|","14.0"
-"`P1908R1 <https://wg21.link/P1908R1>`__","CWG","Reserving Attribute Namespaces for Future Use","Prague","* *",""
 "`P1937R2 <https://wg21.link/P1937R2>`__","CWG","Fixing inconsistencies between constexpr and consteval functions","Prague","* *",""
 "`P1956R1 <https://wg21.link/P1956R1>`__","LWG","On the names of low-level bit manipulation functions","Prague","|Complete|","12.0"
 "`P1957R2 <https://wg21.link/P1957R2>`__","CWG","Converting from ``T*``\  to bool should be considered narrowing (re: US 212)","Prague","* *",""