remove claim of XSI coverage from README

in addition to the dbm functions (which we don't intent to implement
anyway), fmtmsg is still missing too. rather than adding exceptions I
think it's best just to avoid making the claim.
diff --git a/README b/README
index 5b7c70e..dd61bd5 100644
--- a/README
+++ b/README
@@ -13,8 +13,7 @@
 The 1.0 release series for musl features coverage for all interfaces
 defined in ISO C99 and POSIX 2008 base, along with a number of
 non-standardized interfaces for compatibility with Linux, BSD, and
-glibc functionality. Also covered are all XSI interfaces except the
-dbm functions, which are traditionally a separate library from libc.
+glibc functionality.
 
 For basic installation instructions, see the included INSTALL file.
 Information on full musl-targeted compiler toolchains, system