Remove basic-block padding from Wasmtime fuzzing (#6814)

This commit removes the option to generate padding between basic blocks
when fuzzing Wasmtime. Over the weekend lots of OOMs were discovered
related to this option and its most recent update in #6736. The new OOMs
appear to be related to:

* If multiple modules are generated then the configured limits in #6736
  aren't relevant because they only cap one module.
* Each imported function generates a new trampoline which has its own
  set of padding which wasn't previously accounted for.
* Spec tests have a lot of functions and the limits didn't account for
  this.

While each of these is probably individually fixable I think it's
probably not worth the whack-a-mole any more. The `cranelift-fuzzgen`
target should cover the relevant cases for padding without the need for
Wasmtime's fuzzing to cover it as well.

crates/fuzzing/src/generators/config.rs

@@ -208,34 +208,6 @@ impl Config {
             }
         }
 
-        // Try to configure cranelift to insert padding between basic blocks to
-        // stress code layout mechanisms within Cranelift. This padding can get
-        // unwieldy quickly, however, generating a 4G+ function which isn't
-        // particularly interesting at this time.
-        //
-        // To keep this setting under control there are a few limits in place:
-        //
-        // * Cranelift will generate at most 150M of padding per-function,
-        //   regardless of how many basic blocks there are.
-        // * Here it's limited to enable this setting only when there's at most
-        //   10 functions to ensure that the overhead for all functions is <1G
-        //   ish or otherwise doesn't run away.
-        // * The `bb_padding_log2_minus_one` setting isn't allowed to be
-        //   larger than 26 as that allows for `1<<25` maximum padding size
-        //   which is 32M.
-        //
-        // With all that combined this is intended to still be enabled,
-        // although perhaps not all the time, and stress enough interesting test
-        // cases in cranelift.
-        if self.module_config.config.max_funcs < 5 {
-            unsafe {
-                cfg.cranelift_flag_set(
-                    "bb_padding_log2_minus_one",
-                    &(self.wasmtime.bb_padding_log2 % 27).to_string(),
-                );
-            }
-        }
-
         // Vary the memory configuration, but only if threads are not enabled.
         // When the threads proposal is enabled we might generate shared memory,
         // which is less amenable to different memory configurations:
@@ -426,7 +398,6 @@ pub struct WasmtimeConfig {
     native_unwind_info: bool,
     /// Configuration for the compiler to use.
     pub compiler_strategy: CompilerStrategy,
-    bb_padding_log2: u8,
 }
 
 impl WasmtimeConfig {