build system: remove dead presym_enabled? checks

Since presym is now always enabled, the @enable_presym flag,
presym_enabled? method, and all conditional branches guarding
presym-specific code paths are dead code. Remove them and
simplify the affected build logic.

Co-authored-by: Claude <noreply@anthropic.com>

lib/mruby/build.rb

@@ -134,7 +134,6 @@ module MRuby
         @enable_bintest = false
         @enable_test = false
         @enable_lock = true
-        @enable_presym = true
         @enable_benchmark = true
         @mrbcfile_external = false
         @internal = internal
@@ -158,7 +157,7 @@ module MRuby
         if current.libmruby_enabled? && !current.mrbcfile_external?
           current.create_mrbc_build if current.host? || current.gems["mruby-bin-mrbc"]
         end
-        current.presym = Presym.new(current) if current.presym_enabled?
+        current.presym = Presym.new(current)
       end
     end
 
@@ -184,10 +183,6 @@ module MRuby
       @enable_debug = true
     end
 
-    def presym_enabled?
-      @enable_presym
-    end
-
     def disable_lock
       @enable_lock = false
     end
@@ -258,7 +253,7 @@ module MRuby
       if cxx_src
         obj ||= cxx_src + @exts.object
         dsts = [obj]
-        dsts << (cxx_src + @exts.presym_preprocessed) if presym_enabled?
+        dsts << (cxx_src + @exts.presym_preprocessed)
         defines = []
         include_paths = ["#{MRUBY_ROOT}/src", *includes]
         dsts.each do |dst|
@@ -372,7 +367,7 @@ EOS
       end
       [@cc, *(@cxx if cxx_exception_enabled?)].each do |compiler|
         compiler.define_rules(@build_dir, MRUBY_ROOT, @exts.object)
-        compiler.define_rules(@build_dir, MRUBY_ROOT, @exts.presym_preprocessed) if presym_enabled?
+        compiler.define_rules(@build_dir, MRUBY_ROOT, @exts.presym_preprocessed)
       end
     end
 
@@ -558,7 +553,7 @@ EOS
       end
       build.build_mrbc_exec
       build.disable_libmruby
-      build.presym = Presym.new(build) if build.presym_enabled?
+      build.presym = Presym.new(build)
       @mrbc_build = build
       self.mrbcfile = build.mrbcfile
       build

lib/mruby/gem.rb

@@ -96,7 +96,7 @@ module MRuby
 
       def setup_compilers
         (core? ? [@cc, *(@cxx if build.cxx_exception_enabled?)] : compilers).each do |compiler|
-          compiler.define_rules build_dir, @dir, @build.exts.presym_preprocessed if build.presym_enabled?
+          compiler.define_rules build_dir, @dir, @build.exts.presym_preprocessed
           compiler.define_rules build_dir, @dir, @build.exts.object
           compiler.defines << %Q[MRBGEM_#{funcname.upcase}_VERSION=#{version}]
           compiler.include_paths << "#{@dir}/include" if File.directory? "#{@dir}/include"
@@ -119,7 +119,7 @@ module MRuby
       end
 
       def cdump?
-        build.presym_enabled? && @cdump
+        @cdump
       end
 
       def core?