[LangRef] Remove module flag description that does not exist anymore

After https://reviews.llvm.org/D153655, LTOPostLink metadata is not a
thing anymore.

Author: jdoerfert

llvm/docs/LangRef.rst

@@ -7822,13 +7822,6 @@ enum is the smallest type which can represent all of its values::
     !0 = !{i32 1, !"short_wchar", i32 1}
     !1 = !{i32 1, !"short_enum", i32 0}
 
-LTO Post-Link Module Flags Metadata
------------------------------------
-
-Some optimisations are only when the entire LTO unit is present in the current
-module. This is represented by the ``LTOPostLink`` module flags metadata, which
-will be created with a value of ``1`` when LTO linking occurs.
-
 Stack Alignment Metadata
 ------------------------
 

llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll

@@ -2,7 +2,7 @@
 
 ; structs A, B and C have vcall_visibility of public, linkage-unit and
 ; translation-unit respectively. This test is run before LTO linking occurs
-; (the LTOPostLink metadata is not present), so only C can be VFE'd.
+; so only C can be VFE'd.
 
 target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"