Remove representation of floating time from Time

Floating time should not be represented as an instance of `Time` to avoid undefined operations through type safety (see crystal-lang#5332).
This commit removes all implementation details regarding floating time, including the special location `Location::UNSPECIFIED`.

Breaking changes:
* Calls to `Time.new` and `Time.now` are now in the local time zone by
default.
* `Time.parse`, `Time::Format.new` and `Time::Format.parse` don't specify a default location.
  If none is included in the time format and no default argument is provided, the parse method wil raise an exception because there is no way to know how such a value should be represented as an instance of `Time`.
  Applications expecting time values without time zone should provide default location to apply in such a case.

spec/std/time/location_spec.cr

@@ -30,7 +30,6 @@ class Time::Location
         summer_time.offset.should eq 7200
         summer_time.dst?.should be_true
 
-        location.unspecified?.should be_false
         location.utc?.should be_false
         location.fixed?.should be_false
 
@@ -81,12 +80,14 @@ class Time::Location
       end
 
       context "with ZONEINFO" do
-        it "loads from custom source" do
+        it "loads from custom directory" do
           with_zoneinfo(File.join(__DIR__, "..", "data", "zoneinfo")) do
             location = Location.load("Foo/Bar")
             location.name.should eq "Foo/Bar"
           end
+        end
 
+        it "loads from custom zipfile" do
           with_zoneinfo(ZONEINFO_ZIP) do
             location = Location.load("Asia/Jerusalem")
             location.not_nil!.name.should eq "Asia/Jerusalem"
@@ -122,7 +123,6 @@ class Time::Location
       location = Location::UTC
       location.name.should eq "UTC"
 
-      location.unspecified?.should be_false
       location.utc?.should be_true
       location.fixed?.should be_true
 
@@ -137,21 +137,6 @@ class Time::Location
       zone.dst?.should be_false
     end
 
-    it "UNSPECIFIED" do
-      location = Location::UNSPECIFIED
-      location.name.should eq "UNSPECIFIED"
-
-      location.unspecified?.should be_true
-      location.utc?.should be_false
-      location.fixed?.should be_false
-      location.local?.should be_false
-
-      zone = location.lookup(Time.now)
-      zone.name.should eq "UNSPECIFIED"
-      zone.offset.should eq 0
-      zone.dst?.should be_false
-    end
-
     it ".local" do
       with_env("TZ", nil) do
         Location.local.name.should eq "Local"
@@ -171,7 +156,6 @@ class Time::Location
         location.zones.should eq [Zone.new("Fixed", 1800, false)]
         location.transitions.size.should eq 0
 
-        location.unspecified?.should be_false
         location.utc?.should be_false
         location.fixed?.should be_true
         location.local?.should be_false