archive/tar: remore redundant parens in type expressions

Simplify `(T)` expressions to  `T` where possible.

Found using https://go-critic.github.io/overview.html#typeUnparen-ref

Change-Id: Ic5ef335e03898f9fea1ff90fd83956376657fe67
Reviewed-on: https://go-review.googlesource.com/123379
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/archive/tar/format.go

@@ -160,7 +160,7 @@ func (b *block) V7() *headerV7       { return (*headerV7)(b) }
 func (b *block) GNU() *headerGNU     { return (*headerGNU)(b) }
 func (b *block) STAR() *headerSTAR   { return (*headerSTAR)(b) }
 func (b *block) USTAR() *headerUSTAR { return (*headerUSTAR)(b) }
-func (b *block) Sparse() sparseArray { return (sparseArray)(b[:]) }
+func (b *block) Sparse() sparseArray { return sparseArray(b[:]) }
 
 // GetFormat checks that the block is a valid tar header based on the checksum.
 // It then attempts to guess the specific format based on magic values.
@@ -263,7 +263,7 @@ func (h *headerGNU) DevMajor() []byte    { return h[329:][:8] }
 func (h *headerGNU) DevMinor() []byte    { return h[337:][:8] }
 func (h *headerGNU) AccessTime() []byte  { return h[345:][:12] }
 func (h *headerGNU) ChangeTime() []byte  { return h[357:][:12] }
-func (h *headerGNU) Sparse() sparseArray { return (sparseArray)(h[386:][:24*4+1]) }
+func (h *headerGNU) Sparse() sparseArray { return sparseArray(h[386:][:24*4+1]) }
 func (h *headerGNU) RealSize() []byte    { return h[483:][:12] }
 
 type headerSTAR [blockSize]byte
@@ -293,7 +293,7 @@ func (h *headerUSTAR) Prefix() []byte    { return h[345:][:155] }
 
 type sparseArray []byte
 
-func (s sparseArray) Entry(i int) sparseElem { return (sparseElem)(s[i*24:]) }
+func (s sparseArray) Entry(i int) sparseElem { return sparseElem(s[i*24:]) }
 func (s sparseArray) IsExtended() []byte     { return s[24*s.MaxEntries():][:1] }
 func (s sparseArray) MaxEntries() int        { return len(s) / 24 }