wscript: drop unused path_prefix

It is not referenced anywhere, and also not set consistently.

gtk2_ardour/wscript

@@ -13,8 +13,6 @@ def m_hook(self, node):
     """Alias .m files to be compiled the same as .c files, gcc will do the right thing."""
     return self.create_compiled_task('c', node)
 
-path_prefix = 'gtk2_ardour/'
-
 gtk2_ardour_sources = [
         'about.cc',
         'actions.cc',

libs/ardour/wscript

@@ -9,8 +9,6 @@ CURRENT_SESSION_FILE_VERSION = 7002
 
 I18N_PACKAGE = 'ardour'
 
-path_prefix = 'libs/ardour/'
-
 libardour_sources = [
         'amp.cc',
         'analyser.cc',

libs/ardouralsautil/wscript

@@ -5,8 +5,6 @@ import os
 import sys
 import re
 
-path_prefix = 'libs/ardouralsautil/'
-
 def options(opt):
     autowaf.set_options(opt)
 

libs/canvas/wscript

@@ -16,8 +16,6 @@ CANVAS_LIB_VERSION = '0.0.0'
 
 I18N_PACKAGE = 'libcanvas'
 
-path_prefix = 'libs/canvas/'
-
 canvas_sources = [
         'arc.cc',
         'arrow.cc',

libs/clearlooks-newer/wscript

@@ -3,8 +3,6 @@ from waflib.extras import autowaf as autowaf
 import os
 import sys
 
-path_prefix = 'libs/clearlooks-newer'
-
 def options(opt):
     autowaf.set_options(opt)
 

libs/gtkmm2ext/wscript

@@ -47,8 +47,6 @@ gtkmm2ext_sources = [
         'window_title.cc'
 ]
 
-path_prefix = 'libs/gtkmm2ext/'
-
 def options(opt):
     autowaf.set_options(opt)
 

libs/midi++2/wscript

@@ -13,8 +13,6 @@ LIBMIDIPP_VERSION = "%s.%s.%s" % (MAJOR, MINOR, MICRO)
 # micro increment <=> no interface changes
 LIBMIDIPP_LIB_VERSION = '4.1.0'
 
-path_prefix = 'libs/midi++2/'
-
 libmidi_sources = [
         'midi.cc',
         'channel.cc',

libs/pbd/wscript

@@ -19,8 +19,6 @@ LIBPBD_MAJOR_VERSION = '4'
 
 I18N_PACKAGE = 'libpbd4'
 
-path_prefix = 'libs/pbd/'
-
 libpbd_sources = [
     'basename.cc',
     'base_ui.cc',