[Utils] Fix a warning in DialectConversion.cpp

This patch fixes:

  mlir/lib/Transforms/Utils/DialectConversion.cpp:2775:5: error:
  default label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

by removing the default case.  This way, the compiler should issue a
warning in the future when somebody adds a new enum value without a
corresponding case in the switch statement.