This tutorial shows the steps to fix that.
Step 1) Open .properties and remove line with AndroidNature
<natures><nature>com.android.ide.eclipse.adt.AndroidNature</nature><nature>org.eclipse.jdt.core.javanature</nature></natures>
Step 2) remove Android specific buildSpec
<buildSpec><buildCommand><name>com.android.ide.eclipse.adt.ResourceManagerBuilder</name><arguments></arguments></buildCommand><buildCommand><name>com.android.ide.eclipse.adt.PreCompilerBuilder</name><arguments></arguments></buildCommand><buildCommand><name>org.eclipse.jdt.core.javabuilder</name><arguments></arguments></buildCommand><buildCommand><name>com.android.ide.eclipse.adt.ApkBuilder</name><arguments></arguments></buildCommand></buildSpec>
Step 3) Open .classpath and remove android references
<classpathentry kind="con" path="com.android.ide.eclipse.adt.ANDROID_FRAMEWORK"/>
<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.LIBRARIES"/>
<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.DEPENDENCIES"/>
Step 3) Open project.properties (auto-generated) and remove Android references
android.library=true
# Project target.
target=android-17
Step 4) Open .settings/org.eclipse.jdk.core.pref the file should look like that
eclipse.preferences.version=1
org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled
org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.6
org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
org.eclipse.jdt.core.compiler.compliance=1.6
org.eclipse.jdt.core.compiler.debug.lineNumber=generate
org.eclipse.jdt.core.compiler.debug.localVariable=generate
org.eclipse.jdt.core.compiler.debug.sourceFile=generate
org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
org.eclipse.jdt.core.compiler.source=1.6
