FIR: remove unused copy of CONFLICTING_JVM_DECLARATIONS

It was added as a placeholder in f3c58a1df7 and was not used anywhere.
The used error is declared in `JvmBackendErrors` and is reported by the
backend.

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -4919,12 +4919,6 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
-    add(FirJvmErrors.CONFLICTING_JVM_DECLARATIONS) { firDiagnostic ->
-        ConflictingJvmDeclarationsImpl(
-            firDiagnostic as KtPsiDiagnostic,
-            token,
-        )
-    }
     add(FirJvmErrors.OVERRIDE_CANNOT_BE_STATIC) { firDiagnostic ->
         OverrideCannotBeStaticImpl(
             firDiagnostic as KtPsiDiagnostic,

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -3425,10 +3425,6 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         val presentableString: String
     }
 
-    interface ConflictingJvmDeclarations : KtFirDiagnostic<PsiElement> {
-        override val diagnosticClass get() = ConflictingJvmDeclarations::class
-    }
-
     interface OverrideCannotBeStatic : KtFirDiagnostic<PsiElement> {
         override val diagnosticClass get() = OverrideCannotBeStatic::class
     }

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -4130,11 +4130,6 @@ internal class PreReleaseClassImpl(
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<PsiElement>(firDiagnostic, token), KtFirDiagnostic.PreReleaseClass
 
-internal class ConflictingJvmDeclarationsImpl(
-    firDiagnostic: KtPsiDiagnostic,
-    token: KtLifetimeToken,
-) : KtAbstractFirDiagnostic<PsiElement>(firDiagnostic, token), KtFirDiagnostic.ConflictingJvmDeclarations
-
 internal class OverrideCannotBeStaticImpl(
     firDiagnostic: KtPsiDiagnostic,
     token: KtLifetimeToken,