remove rationale from index

Author: Moxinilian

mlir/docs/Rationale/_index.md

@@ -29,7 +29,3 @@ rationale for some of the design decisions behind MLIR.
 [Usage of 'const' in MLIR, for core IR types](UsageOfConst.md)
 :   Explains the rationale for eschewing the use of `const` entirely for the
     core IR types in MLIR.
-
-[IRDL Rationale](IRDLRationale.md)
-:   Describes the purpose of the IRDL dialect and the guiding principles behind
-    it.