Remove the leading fire from the taskbar progress handler (#19403)

If the progress state hasn't been set for more than 200ms, we shouldn't
even bother flickering the old state.

This prevents applications from making the tab (and the taskbar icon)
flicker.

We were reviewing #19394 and decided that the _original_ behavior before
Leonard's throttling fix was somewhat unfortunate as well. An
application that sets an indeterminate state for 10ms and then clears it
shouldn't be able to make any part of the application flicker, fast _or_
slow.

Removing the leading fire time from the throttled function ensures that
it will only fire once every 200ms, and only with the state most
recently set. It will not debounce (so setting the progress every 150ms
will not prevent it from updating.)

Closes #19394

(cherry picked from commit 998ab58)
Service-Card-Id: PVTI_lADOAF3p4s4AxadtzgfZOsM PVTI_lADOAF3p4s4Axadtzgfb1Nw
Service-Version: 1.23

Author: DHowett

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -973,7 +973,6 @@ namespace winrt::TerminalApp::implementation
             dispatcher,
             til::throttled_func_options{
                 .delay = std::chrono::milliseconds{ 200 },
-                .leading = true,
                 .trailing = true,
             },
             [weakThis]() {