Remove viewer_strings.properties (#881)

It was only used be the old ui viewer, which was removed with #1431 already

pmd-core/src/main/resources/net/sourceforge/pmd/util/viewer/resources/viewer_strings.properties

@@ -1,38 +0,0 @@
-#
-# BSD-style license; for more info see http://pmd.sourceforge.net/license.html
-#
-
-##########################################################################
-# English string translations for the viewer
-#
-##########################################################################
-
-# Main Frame
-MAIN.FRAME.TITLE                        = AST Viewer
-MAIN.FRAME.COMPILE_BUTTON.TITLE         = Compile
-MAIN.FRAME.EVALUATE_BUTTON.TITLE        = Evaluate XPath Expression
-MAIN.FRAME.COMPILATION.TOOK             = Compilation took
-MAIN.FRAME.COMPILATION.PROBLEM          = Compilation problem:
-MAIN.FRAME.EVALUATION.TOOK              = Evaluation took
-MAIN.FRAME.EVALUATION.PROBLEM           = Evaluation problem:
-
-
-# Source Panel
-SOURCE.PANEL.TITLE                      = Source code
-
-# Tree Panel
-AST.PANEL.TITLE                         = Abstract Syntax Tree
-AST.MENU.ATTRIBUTES                     = Add predicate from {0}'a attributes
-AST.MENU.NODE.TITLE                     = Add {0} to the XPath expression
-AST.MENU.NODE.ADD_ABSOLUTE_PATH         = Add with absolute path
-AST.MENU.NODE.ADD_ALLDESCENDANTS        = Add all descendants of this type
-
-# XPath Panel
-XPATH.PANEL.TITLE                       = XPath
-XPATH.PANEL.EXPRESSION                  = Expression
-XPATH.PANEL.RESULTS                     = Results
-
-# Compile Error dialog
-COMPILE_ERROR.DIALOG.TITLE              = Compilation error
-COMPILE_ERROR.PANEL.TITLE               = Error message
-COMPILE_ERROR.OK_BUTTON.CAPTION         = Ok