wscript: drop unused APPNAME assignment in libs

https://waf.io/book/ says
  By default, the project name and version are set to noname and 1.0. To
  change them, it is necessary to provide two additional variables in
  the top-level project file

- and waf code inspection confirms that waf itself only will use the top
level APPNAME.

Also, the 'waf dist' comment doesn't seem relevant - especially after
this change - and is removed too.

(Note: libs/evoral/wscript and libs/temporal/wscript still use APPNAME
for other purposes.)

libs/pbd/wscript

@@ -17,8 +17,6 @@ LIBPBD_VERSION = "%s.%s.%s" % (MAJOR, MINOR, MICRO)
 LIBPBD_LIB_VERSION = '4.1.0'
 LIBPBD_MAJOR_VERSION = '4'
 
-# Variables for 'waf dist'
-APPNAME = 'libpbd'
 I18N_PACKAGE = 'libpbd4'
 
 # Mandatory variables