test(appsec): trim redundant decoder tests from FileItemContentReaderTest

Charset fallback scenarios are covered by MultipartContentDecoderTest.
One integration test is kept to verify that getContentType() is passed through.

Author: jandro996

dd-java-agent/instrumentation/commons-fileupload-1.5/src/test/groovy/FileItemContentReaderTest.groovy

@@ -39,7 +39,7 @@ class FileItemContentReaderTest extends Specification {
     FileItemContentReader.readContent(item) == ''
   }
 
-  void 'readContent decodes UTF-8 when Content-Type specifies charset=UTF-8'() {
+  void 'readContent uses Content-Type from file item for charset decoding'() {
     given:
     def text = 'héllo wörld'
     def item = fileItemFromBytes(text.getBytes('UTF-8'), 'file.txt', 'text/plain; charset=UTF-8')
@@ -48,34 +48,6 @@ class FileItemContentReaderTest extends Specification {
     FileItemContentReader.readContent(item) == text
   }
 
-  void 'readContent falls back to UTF-8 when Content-Type has no charset'() {
-    given:
-    def text = 'hello world'
-    def item = fileItemFromBytes(text.getBytes('UTF-8'), 'file.txt', 'text/plain')
-
-    expect:
-    FileItemContentReader.readContent(item) == text
-  }
-
-  void 'readContent falls back to UTF-8 when Content-Type is null'() {
-    given:
-    def text = 'hello world'
-    def item = fileItemFromBytes(text.getBytes('UTF-8'), 'file.txt', null)
-
-    expect:
-    FileItemContentReader.readContent(item) == text
-  }
-
-  void 'readContent falls back to ISO-8859-1 when bytes are invalid UTF-8'() {
-    given:
-    // 0xE9 is 'é' in ISO-8859-1 but an invalid lone UTF-8 byte
-    byte[] iso88591Bytes = 'café'.getBytes('ISO-8859-1')
-    def item = fileItemFromBytes(iso88591Bytes, 'file.txt', null)
-
-    expect:
-    FileItemContentReader.readContent(item) == 'café'
-  }
-
   void 'readContents returns content for each non-form file with a name'() {
     given:
     def items = [fileItem('content-a', 'file-a.txt'), fileItem('content-b', 'file-b.txt'),]